In industries, cone crushers are used in crushing big stones into small pieces for use in construction. FIGS. 1 and 2 show a hydraulic cone crusher designed by the present inventor. The hydraulic cone crusher generally comprises a bottom frame 2, a top frame 3, a main shaft 4, and an eccentric 5. A counter shaft 21 is mounted in the bottom frame 2 at one side. The counter shaft 21 has one end mounted with pinion gear 22, and an opposite end extended out of the bottom frame 2 and mounted with a pulley 23 for coupling to a motor. The top frame 3 is fixedly mounted on the bottom frame 2 at the top, having a concave 31 on the inside, a feed hopper 32 and a distributor 33 on the top. The distributor 33 is driven by a motor 1 mounted on a rack 321 at one side of the feed hopper 32. The main shaft 4 is vertically disposed inside the top frame 3 and the bottom frame 2 at the center. The top end of the main shaft 4 is supported in a locating block 35. The locating block 35 holds a spider bearing 36', a bearing block 36 and a shaft sleeve 37. A mantle core 40 is mounted around the main shaft 4 on the middle. A mantle 41 is mounted on the tapered wall of the mantle core 40. A check nut 401 is disposed above the mantle 41. The eccentric 5 is mounted around the lower part of the main shaft 4, having an outward coupling portion 51 at the bottom side thereof supported on a hydraulic cylinder 6. A big bevel gear 52 is fixedly mounted on the outward coupling portion 51 of the eccentric 5, and meshed with the pinion gear 22 on the counter shaft 21. The hydraulic cylinder 6 is provided at the bottom of the bottom frame 2, having a top opening through which the main shaft 4 is inserted into the hydraulic cylinder 6, and an outward top flange 60 around its top opening. The outward top flange 60 is mounted with a copper plate 601 for supporting the eccentric 5. A piston 61 is reciprocated in the hydraulic cylinder 6. The piston 61 is mounted with a first thrust board 62, and a second thrust board 63 above the first thrust board 62. The main shaft 4 has a bottom end mounted with a thrust board 42 supported on the second thrust board 63. The bottom end of the hydraulic cylinder 6 is connected to a hydraulic fluid pump (not shown) through a pipe 64. When the eccentric 5 is rotated during the operation of the counter shaft 21, the main shaft 4 is driven to rotate by the eccentric 5 and the inner bushing 54 in the eccentric 5. By means of reciprocating the piston 61, the main shaft 4 is moved vertically in the inner bushing 54 to adjust the crushing gap between the mantle 41 and the concave 31 without stopping the machine. As indicated above and shown in FIG. 2, the main shaft 4 is mounted inside the top frame 2 and the bottom frame 3 at the center. The top end of the main shaft 4 is supported on the locating block 35. The locating block 35 has mounted therein a fixed bearing block 36, a spider bearing 36' and a shaft sleeve 37 for supporting the top end of the main shaft 4 in place, permitting the main shaft 4 to be smoothly rotated by the eccentric 5. This arrangement still has drawbacks. Because the spider bearing 36' wear quickly with use, the gap between the spider bearing 36' and the main shaft 4 increases with the wearing condition of the spider bearing 36'. When the spider bearing 36' wears gradually, the rotary motion of the main shaft 4 becomes unstable, thereby causing a damage to the main shaft 4. Therefore, the spider bearing 36' must be frequently replaced before it starts to wear.